Effect of Alkali and Trivalent Metal Ions on the High-Pressure Phase Transition of [C₂H₅NH₃]M^I_0.5M^III_0.5(HCOO)₃ (M^I = Na, K and M^III = Cr, Al) Heterometallic Perovskites

We report the high-pressure behavior of two perovskite-like metal formate frameworks with the ethylammonium cation (EtAKCr and EtANaAl) and compare them to previously reported data for EtANaCr. High-pressure single-crystal X-ray diffraction and Raman data for EtAKCr show the occurrence of two high-pressure phase transitions observed at 0.75(16) and 2.4(2) GPa. The first phase transition involves strong compression and distortion of the KO₆ subnetwork followed by rearrangement of the −CH₂CH₃ groups from the ethylammonium cations, while the second involves octahedral tilting to further reduce pore volume, accompanied by further configurational changes of the alkyl chains. Both transitions retain the ambient P2₁/n symmetry. We also correlate and discuss the influence of structural properties (distortion parameters, bulk modulus, tolerance factors, and compressibility) and parameters calculated by using density functional theory (vibrational entropy, site-projected phonon density of states, and hydrogen bonding energy) on the occurrence and properties of structural phase transitions observed in this class of metal formates.